Re: [PATCH v1 05/39] mm/rmap: introduce and use hugetlb_try_share_anon_rmap()

[Ryan Roberts]

On 11/12/2023 15:56, David Hildenbrand wrote:
> hugetlb rmap handling differs quite a lot from "ordinary" rmap code.
> For example, hugetlb currently only supports entire mappings, and treats
> any mapping as mapped using a single "logical PTE". Let's move it out
> of the way so we can overhaul our "ordinary" rmap.
> implementation/interface.
> 
> So let's introduce and use hugetlb_try_dup_anon_rmap() to make all
> hugetlb handling use dedicated hugetlb_* rmap functions.
> 
> Note that try_to_unmap_one() does not need care. Easy to spot because
> among all that nasty hugetlb special-casing in that function, we're not
> using set_huge_pte_at() on the anon path -- well, and that code assumes
> that we would want to swapout.
> 
> Reviewed-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>

> ---
>  include/linux/rmap.h | 23 +++++++++++++++++++++++
>  mm/rmap.c            | 15 ++++++++++-----
>  2 files changed, 33 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index ca42b3db5688..4c0650e9f6db 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -228,6 +228,29 @@ static inline int hugetlb_try_dup_anon_rmap(struct folio *folio,
>  	return 0;
>  }
>  
> +/* See page_try_share_anon_rmap() */
> +static inline int hugetlb_try_share_anon_rmap(struct folio *folio)
> +{
> +	VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio);
> +	VM_WARN_ON_FOLIO(!PageAnonExclusive(&folio->page), folio);
> +
> +	/* Paired with the memory barrier in try_grab_folio(). */
> +	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP))
> +		smp_mb();
> +
> +	if (unlikely(folio_maybe_dma_pinned(folio)))
> +		return -EBUSY;
> +	ClearPageAnonExclusive(&folio->page);
> +
> +	/*
> +	 * This is conceptually a smp_wmb() paired with the smp_rmb() in
> +	 * gup_must_unshare().
> +	 */
> +	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP))
> +		smp_mb__after_atomic();
> +	return 0;
> +}
> +
>  static inline void hugetlb_add_file_rmap(struct folio *folio)
>  {
>  	VM_WARN_ON_FOLIO(folio_test_anon(folio), folio);
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 4e60c1f38eaa..e210ac1b73de 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -2147,13 +2147,18 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
>  				       !anon_exclusive, subpage);
>  
>  			/* See page_try_share_anon_rmap(): clear PTE first. */
> -			if (anon_exclusive &&
> -			    page_try_share_anon_rmap(subpage)) {
> -				if (folio_test_hugetlb(folio))
> +			if (folio_test_hugetlb(folio)) {
> +				if (anon_exclusive &&
> +				    hugetlb_try_share_anon_rmap(folio)) {
>  					set_huge_pte_at(mm, address, pvmw.pte,
>  							pteval, hsz);
> -				else
> -					set_pte_at(mm, address, pvmw.pte, pteval);
> +					ret = false;
> +					page_vma_mapped_walk_done(&pvmw);
> +					break;
> +				}
> +			} else if (anon_exclusive &&
> +				   page_try_share_anon_rmap(subpage)) {
> +				set_pte_at(mm, address, pvmw.pte, pteval);
>  				ret = false;
>  				page_vma_mapped_walk_done(&pvmw);
>  				break;